Pumps of this type are known which are provided with a hollow shaft and used for instance as circulating pumps in central-heating plants. These pumps are, however, encumbered with the draw-back that the shaft is relatively expensive to manufacture because it is made of a solid material or a relatively thick-walled tube by cutting, which involves relatively high manufacturing costs for the shaft.
German Offenlegungsschrift No. 2447542 discloses furthermore the use of a hollow shaft in the form of a relatively thin-walled tube for relatively small, simple, and inexpensive pumps including a conventional electromotor. These pumps are, however, encumbered with the draw-back that they cannot transfer a high torque between the shaft and the impeller. Unlike the hollow shaft of a pump comprising a canned motor the hollow shaft of the above pump has no technical function beyond the transfer of the torque to the impeller.
A desire arises for providing a valve in the inner openings of the hollow shaft, said valve preventing the pump fluid from returning to the suction side through the hollow shaft, which causes a reduction of the efficiency of the pump. When the vent plug in the end wall of the motor housing is removed at inspection of the direction of rotation of the pump, the above valve prevents the pump fluid from flowing from the suction side upwards through the hollow shaft.